Sex and sexual activities between consenting adults are no business of the state, and should be legal regardless of who the persons involved are, what their connection to each other may be or what their relationship to each other may be.

Examples:-
Doctor/patient relations
Teacher/student relations (where the student is above the age of consent)
Incestual relations (so long as adequate contraception is used)

In cases where two adults consent, it is not for the state to decide that one or both of these people is or are acting improperly.